1. Field of the Invention
The present invention relates to polymer microparticles prepared by heterogeneous polymerization of a monomer in a supercritical fluid and/or subcritical fluid, and a production method for the same. The polymer microparticles promise to be used as constituent materials of developers in electrophotography, printing inks, building paints, and cosmetics.
2. Description of the Related Art
There have been proposed microparticle production methods that involve heterogeneous polymerization of a monomer in supercritical carbon dioxide, and emulsion polymerization, dispersion polymerization, suspension polymerization, etc., are well known in the art. Among other methods, heterogeneous polymerization conducted in supercritical carbon dioxide is advantageous over conventional heterogeneous polymerization conducted in water or organic solvent, since it can (1) achieve simplification of solvent removal (drying) step after polymerization, (2) requires no waste solvent treatment, and (3) uses no toxic organic solvents. For these reasons, heterogeneous polymerization is widely used for the preparation of microparticles from a monomer, and the resultant microparticles are used for instance in the above-described applications. Nevertheless, many of the conventional preparation methods involving heterogeneous polymerization in supercritical carbon dioxide require surfactant upon granulation; therefore, an optional surfactant need to be prepared in advance for each type of the monomer to be used. When polymer particles are to be prepared from multiple types of monomers by heterogeneous polymerization, different surfactants need to be previously prepared for different types of monomers, requiring multiple devices and resulting in prolonged lead time, increased production steps, and low yields. These disadvantages increase production costs, and therefore, there remains a need in the art to overcome the disadvantages.
Specifically, the first objective in the art was to obtain polymer particles without having to prepare different surfactants for different types of monomers.
The second objective was to improve compatibility between the obtained particles and organic medium, resin, metal, etc. Poor compatibility is attributed to the fact that the obtained particles have substantially smooth surface and are substantially spherical. Thus, there often occurs a situation where polymer particles combined with organic medium for use as a film or paint come off from the medium, a situation where resin or metal covering the particle surface easily come off, and so forth.
To achieve the first objective, Japanese Patent Application Laid-Open (JP-A) No. 2002-179707 discloses a method of preparing submicron resin particles by polymerization of an acrylic monomer while using a polymerization initiator having a polydimethylsiloxane skeleton (product name: VPS-501 (Wako Pure Chemical Industries, Ltd.)) This method, however, is encountered with difficulty in obtaining discrete particles since they undergo flocculation and aggregation. In particular, it has been difficult with this method to obtain micron resin particles since they tend to be flocculated and aggregated extensively. Moreover, polymerization particles obtained with this method generally have a weight-average molecular weight (Mw) of 100,000 to 600,000 and a number-average molecular weight (Mn) of 50,000 to 300,000. Thus, with this method, it has been difficult to obtain low-molecular-weight polymerization particles with a molecular weight of around 3,000 to 50,000 suitable for use as toner or image forming particles. The reason for this is that low-molecular-weight polymerization particles are prone to fluidization due to plasticization by means of supercritical carbon dioxide and thus are more likely to undergo flocculation and aggregation than high-molecular-weight polymerization particles. Namely, it has been very difficult to prepare micron polymerization particles with a low molecular weight without causing flocculation and aggregation.
As another approach to achieve the above first objective, Shishido et al of Yamagata University proposes, on page 152 of their book titled “Supercritical Fluid and Nanotechnology,” a method of preparing polymer particles by using, without any surfactant, acrylonitrile as a monomer in which the obtained polymerization particles are insoluble. In this method, however, unwanted particle flocculation occurs due to the absence of surfactant. Moreover, this method is significantly limited in applicability since it is required to exploit the nature of the resultant polymerized particles that they are insoluble in the monomer used; therefore, employable monomers are limited to acrylonitrile, etc, and this method cannot be used for the production of general-purpose polymers such as polystyrene and methyl methacrylate (MMA). U.S. Pat. No. 5,552,502 issued to Odell et al studies on deposition polymerization in supercritical carbon dioxide containing sulfur dioxide. However, sulfur dioxide presents safety problem since it is toxic and corrosive to the device. U.S. Pat. No. 5,688,870 issued to Wilkinson et al tries to prepare particles with improved water dispersibility by preparing resin particles from a polymerizable monomer in supercritical carbon dioxide using a silicone surfactant and by forming a hydrophilic shell layer on the particle surface. However, this method requires in-advance preparation of surfactants, i.e., separate steps for development and preparation of surfactants optimal for the intended type of polymer particles (i.e., monomer types). Thus, it is quite challenge to improve the above-noted production process.
To achieve the second objective attempts have been made to modify particle surface for increased chemical affinity, but have met with limited success. Another possible physical method for obtaining anchor effects on the particle surface is to obtain particles by pulverization. However, pulverization results in generation of particles with a broad particle size distribution—from coarse particles to finely divided particles. This makes classification indispensable, which is undesirable in view of productivity and costs. Also, pulverization may result in too large variations in particle shape.
In-water polycondensation has been contemplated as a method of improving the anchor effect by roughening the particle surfaces for increased specific surface areas. This method, however has disadvantages such as low monomer selectivity and generation of large volumes of waste water.